Read the passage carefully and answer the questions based on the passage:

The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction. In the rate equation Rate = $k [A]^x [B]^y$

$x$ and $y$ indicate how sensitive the rate is to the change in concentration of A and B, respectively. Sum of these exponents, i.e., $x + y$ gives the overall order of a reaction where $x$ and $y$ represent the order with respect to the reactants A and B, respectively. Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. For a first order reaction, the concentration of the reactant varies as $[R] = [R]_0e^{-kt}$

Which of the following is a zero order reaction?

Decomposition of gaseous ammonia on a hot platinum surface

The correct answer is Option (3) → Decomposition of gaseous ammonia on a hot platinum surface

Here's why:

• Zero-Order Reactions have a rate that is independent of the concentration of the reactant(s). This often happens when the reaction occurs on a catalyst surface, and the surface area is fully saturated by reactant molecules.
• Decomposition of gaseous ammonia ($\text{NH}_3$) on a hot platinum surface ($3\text{NH}_3(\text{g}) \rightarrow{\text{Pt}} \text{N}_2(\text{g}) + 3\text{H}_2(\text{g})$):
• This is a heterogeneous reaction where the rate is controlled by the surface area of the platinum catalyst.
• At high ammonia concentrations, the catalyst surface becomes completely covered (saturated) with $\text{NH}_3$ molecules.
• Once saturated, increasing the concentration of $\text{NH}_3$ in the gas phase does not increase the number of molecules reacting per unit time on the surface.
• Therefore, the rate remains constant, making it a zero-order reaction.

Analysis of Other Options:

1. Radioactive decay of unstable nuclei: This follows first-order kinetics. The rate depends only on the amount of the radioactive substance present ($\text{Rate} = k[\text{N}]$).
2. Hydrogenation of ethene ($\text{C}_2\text{H}_4$): This reaction, typically carried out over a metal catalyst, often follows complex kinetics but is usually not purely zero order; it may be zero order with respect to $\text{H}_2$ at high pressure but often first order with respect to ethene.
3. Decomposition of $\text{N}_2\text{O}_5$ ($2\text{N}_2\text{O}_5(\text{g}) \rightarrow 4\text{NO}_2(\text{g}) + \text{O}_2(\text{g})$): This is a well-known example of a homogeneous first-order reaction.